It is known that it is difficult to fabricate parts of large size made of ceramic, in particular made of SiC. The tolerances after sintering the primary components made of silicon carbide of large size are ill controlled and the machining of these components is unacceptable for cost-related reasons.
In addition, and for the same reasons, it is generally difficult to fabricate parts of complex shape with silicon-based compounds such as silicon carbide.
It is therefore often preferable to fabricate parts or structures of large size and/or of complex shape from elements in ceramic of simple shape and/or of small size, and then to assemble these elements to form the final structure.
Said technique is particularly necessary for fabricating structures of heat exchanger type and structural components in silicon carbide having a temperature of use possibly reaching up to for example 900° C., even 1000° C.
On account of the high temperatures, close to 900° C. to 1000° C. for example, used in applications of ceramics such as silicon carbide, the joining of these ceramics by bonding with organic adhesives is excluded since the temperatures of use of this type of assembly cannot exceed 200° C. at the most.
Purely mechanical assemblies, for example by stapling or screwing, only ensure partial, random contact between the parts. The assemblies thus obtained cannot be leak tight. The mechanical strength is only ensured by the staples and screws, which is limited. To ensure good mechanical strength of the joint, it is essential to create good adhesion between the parts to be joined, which is not possible with screws or staples.
Additionally, conventional joining techniques by welding having recourse to an energy beam with or without a supply of metal (TIG, electron or laser welding) and involving the partial melting of the parts to be joined cannot be used for assembling ceramics since it is not possible to melt a substrate or a part in ceramic, and in particular since silicon carbide decomposes before melting.
Usual techniques for obtaining refractory assemblies of ceramics are solid phase diffusion bonding and joining by sintering or co-sintering.
For assembly by diffusion bonding, a pressure is applied at high temperature between the interfaces to allow atomic inter-diffusion between the two substrates. The temperature must always remain lower than the melting point of the least refractory material, and there is therefore no liquid phase in the system. This type of joining is obtained either under a press in single direction, or in an isostatic chamber. Diffusion bonding is well adapted for the joining of two metal alloys and very little adapted for the joining of ceramic materials, since the atoms forming the ceramic scarcely diffuse at the joint. In addition, the method is prohibitive from a mechanical viewpoint since it requires placing under compression porous, fragile substrates and materials such as silicon carbide composites which risk being highly damaged under this mechanical compressive loading.
The joining by sintering or co-sintering of parts made of SiC also requires high pressures but in addition high temperatures and long hold times since this process is based on the principle of inter-diffusion between the SiC elements.
In other words, solid phase diffusion bonding and joining by sintering have the disadvantage of being restrictive from an implementation standpoint since:                for solid phase diffusion bonding, the shape of the parts must remain simple if uniaxial pressing is used, or else it requires complex tooling and preparation for example entailing the fabrication of a jacket, vacuum sealing, hot isostatic pressing, final machining of the jacket if HIP is used (Hot Isostatic Pressing).        for co-sintering or joining by sintering the problems remain the same (shape of the parts, complex implementation) with, in addition, the need to control the sintering of a filler powder to be inserted between the two materials to be joined.        these two techniques additionally require the use of long hold times (one to several hours) at high temperature since the processes used have recourse to solid state diffusion.        
It follows from the above, and to summarize, that in order to guarantee good mechanical strength in particular and optionally satisfactory sealing of the assembly, only those processes using a liquid phase such as brazing can be envisaged.
Brazing is a low-cost technique, easy to perform and is the most commonly used. Parts of complex shape can be prepared using brazing, and brazing operations are limited to placing between the parts to be joined, or in the vicinity of the joint between the two parts, a filler alloy called a braze alloy and melting this alloy which is capable of wetting and spreading over the interfaces to be joined, filling the joint between the parts. After cooling the brazing alloy solidifies ensuring the cohesion of the assembly.
Most brazing compositions for parts in silicon carbide-based materials are insufficiently refractory. These are generally brazing compositions formed by metal alloys having a melting point that is lower even much lower than 1000° C. Said melting temperature is distinctly insufficient for applications at temperatures in the region of 900° C. or 1000° C., for example from 950° C. to 980° C.
Also, most chemical elements which form part of these metal brazing compositions are highly reactive with silicon carbide on and after 500° C. and lead to fragile compounds.
As a result, for brazing at higher temperatures generally above 1000° C., said brazing compositions or braze alloys would chemically attack the silicon carbide-based materials not only during the brazing operation but also during functional use by solid state diffusion.
It is also pointed out that the least reactive alloys are also the least refractory, such as the AgCuTi alloy for example with Ag—Cu matrix and active Ti element in low concentration. For the applications more particularly concerned by the invention, which are those of moderately refractory assemblies having a temperature of use of generally up to 950° C., even 980° C., all the reactive brazing compositions chiefly containing silver, or silver-copper, copper, nickel, iron or cobalt, platinum, palladium or gold are therefore to be excluded on account of their strong reactivity with silicon carbide.
Formulations of brazing alloys, brazing compositions, that are more refractory and with high silicon content are presented in documents [1, 2, 3]. These brazing compositions have scarcely reactive behaviour, even non-reactive, with SiC which prevents the formation of fragile compounds. This criterion of non-reactivity or very low reactivity is not a sufficient condition however for guaranteeing good mechanical strength of the brazed joints. In the literature, the yield strength values of binary silicon-based brazing alloys are most variable in relation to the second element taking part in the silicon-based non-reactive brazing composition.
For example, for the non-reactive Fe—Si system (45% Fe-55% Si by weight), document [3] mentions an extremely low ultimate tensile strength of the order of 2 MPa, despite the non-reactivity of this composition indicated in document [4], whilst for the Cr—Si system (25% Cr-75% Si by weight), this same document [3] gives a higher value of the order of 12 MPa.
For a non-reactive Co—Si alloy (90% Si-10% Co by weight), document [1] mentions a value of about 100 MPa under compression/shear.
The properties, in particular mechanical properties, of a silicon-based brazing composition are fully unpredictable and can absolutely not be inferred from the mechanical properties of already known Si-based brazing compositions, even if of very close type.
In other words, when it is sought to prepare a silicon-based brazing composition in particular for brazing parts in SiC, it is absolutely not possible to refer to the mechanical properties which may be acceptable exhibited by other known Si-based brazing compositions, since any modification however small of a Si-based brazing composition whether concerning the type of the metal(s) brazed with the silicon or the proportions thereof, may lead to unpredictable, unexpected even major changes in the properties of the composition and in particular its mechanical properties.
To conclude, it is not possible to predict the mechanics of a given binary Si—X system where X is a metal, and even less so the mechanics of a said system as a function of the proportions of X. For all the more reason, it is impossible to predict the mechanics of a more complex system such as a ternary Si—X—Z system where X and Z are metals.
The brazing temperatures of the brazing compositions in documents [1, 2] and [3] are generally higher than 1300° C. These brazing temperature are for example 1355° C. for the Ti—Si composition (22-78% by weight), 1355° C. for the Cr—Si composition (25-75% by weight), 1400° C. to 1450° C. for the Co—Si composition, and 1750° C. for the Ru2Si3 composition.
The efficacy of this joining method requires brazing temperatures higher than 1300° C. for thermodynamic destabilization of the passivating silicon oxide layers which occur spontaneously on the silicon carbide surfaces, since these silicon oxide layers are detrimental to wetting by the brazing composition, even if brazing is conducted in a vacuum.
Therefore the above-mentioned brazing alloys with high silicon content and used at a temperature higher than 1300° C. are not suitable for the brazing of substrates in silicon carbide-based materials whose properties are degraded after exposure to 1300° C., even more so for those which degrade at 1150° C., even 1100° C. or lower. This is notably the case with some SiC/SiC composites which degrade at above 1300° C., even 1150° C., and even at above 1100° C. such as the CMC examined in the Examples which degrades on and after 1100° C.
It is true that document [3] in Example 2 presents a Ni—Si brazing composition (65% Ni-35 Si % by weight, i.e. 47 atomic % Ni-53 atomic % Si) which can be brazed at 1120° C., for 16 hours. This brazing temperature is slightly higher than the preferred brazing temperature used in the invention which is 1100° C., but it uses a very long brazing hold time. However, the mechanical strength of the joint obtained with this composition (ultimate tensile strength of 375 p.s.i.—i.e. about 2.6 MPa) is very low despite the non-reactivity of this composition mentioned in document [5]. This mechanical strength is insufficient for numerous applications and in particular the main applications concerned herein, despite the low reactivity of this brazing composition with SiC.
It is also to be pointed out that this Ni—Si brazing alloy (65 wt %-35 wt %) has a melt onset temperature of 966° C. (eutectic at 966°) which is not suitable for applications at 950° C.-980° C.
For higher Si contents, it is specified in document [5] that Ni—Si brazing alloys are not reactive, but no mechanical data is provided. The work described in document [5] focuses on the study of wetting angles and the work of adhesion (thermodynamic adhesion at a solid/liquid interface, this adhesion is defined by the work needed for reversible separation of a solid/liquid interface into two solid/vapour and liquid/vapour surfaces. Finally, it is noted that for these Ni—Si brazing alloys, the range between liquidus and solidus is very extensive with, as already mentioned above, the onset of melting on and after 966° C. (for Ni 66% by weight, but also for close-lying contents due to the presence of an eutectic at 966° C.) which limits application temperatures to below 900° C.
Document [6] mentions a brazing alloy Ni-13.4 Cr-40 Si (atomic %) whose melting point is 1150° C. and which is used at a brazing temperature of 1200° C. The authors did not conduct mechanical characterization on the brazed joints and only metallurgical characteristics are given which indicate non-reactivity.
No mechanical test result on this alloy is provided which means that good mechanical strength of the brazing can in no way be guaranteed.
Document [2] proposes (Example 3) a Pt—Si alloy which is brazed at 1200° C. The Pt content of this brazing composition is very high (77 weight % Pt), which leads to a very costly process. This disadvantage is prohibitive for the obtaining of large-size brazed parts.
At all events, 1200° C. is a temperature that is too <<refractory>> for the applications concerned by the present invention.
Finally document [7] presents brazing alloys having a Si content of less than 50 weight %, preferably 10 to 45 weight %, and with the addition of at least 2 elements chosen from the following group: Li, Be, B, Na, Mg, P, Sc, Ti, V, Cr, Mn, Fe, Co, Zn, Ga, Ge, As, Rb, Y, Sb, Te, Cs, Pr, Nd, Ta, W and Ti. In this group of elements at least one thereof is preferably a metal chosen from among Fe, Cr, Co, V, Zn, Ti and Y. Neither nickel nor aluminium are cited.
The examples in document [7] describe ternary brazing compositions: Si—Cr—Co (11:38.5:50.5% by weight); Si—Cr—Co (40:26:34% by weight); Si—Fe—Cr (17.2:17.5:65.3% by weight); and Si—Fe—Co (20:20:60% by weight); and the brazing thereof at temperatures respectively of 1230° C., 1235° C., 1460° C. and 1500° C.
The brazing compositions in document [7] never contain the nickel element or aluminium element.
Regarding the brazing compositions having brazing temperatures lower than 1300° C., it is simply mentioned that a <<strong>> bond is obtained and no mechanical test is provided to prove that good mechanical strength of the joints is effectively obtained. Also, the low reactivity of the SiC/brazing filler is neither mentioned nor referred to.
In the light of the foregoing there is therefore a need, not yet met, for a method with which it is possible to obtain the joining by brazing of parts in silicon carbide-based materials, more specifically of moderately refractory substrates in silicon carbide, which ensures satisfactory mechanical strength of the assembly at between 20° C. and 950° C. even 980° C., in particular above 500° C. and up to 950° C. even 980° C., and optionally also sealing of the joint.
This method must allow the use in particular of brazing temperatures equal to or lower than 1150° C. and preferably of 1100° C. which is a temperature that it is absolutely essential not to exceed for some SiC-based substrates, parts to be joined.
It is effectively essential that the parts, substrates maintain their full integrity and initial performance levels after the joining operation by brazing.
There is therefore a need for a brazing method using brazing compositions which allows the desired temperatures of use to be reached namely up to 950° C. even 980° C., whilst avoiding the subjecting of the parts, substrates in silicon carbide-based materials to temperature ranges which could deteriorate these materials.
In other words, there is a need for a brazing method which allows moderately refractory brazed joints to be obtained (with a temperature of use of up to about 950° C. even 980° C.) using brazing cycles generally not exceeding a limit temperature defined between 1040° C. and 1150° C., in particular of 1100° C. depending on the SiC-based material to be joined.
Numerous silicon carbide-based materials, in particular some composites, are irreversibly deteriorated over and above 1100° C.: this is particularly the case with some composites formed of a SiC matrix and SiC fibres such as the composite available from SNECMA Propulsion Solide under the trade name Cerasep A40C®.
In addition, the holding time of the brazing plateau at a temperature equal to or lower than 1150° C., for example of 1100° C., must preferably be from one or a few minutes to two or three hours at most to avoid degradation of the composite.
On the other hand, pure silicon carbide withstands brazing at 1450° C.
In other words, there is a need for a brazing method and composition, brazing alloy, firstly allowing the use of the full refractory potential of the silicon carbide-based substrates at temperatures of use of up to about 950° C. even 980° C., and secondly allowing brazing at a brazing temperature lower than the degradation temperature of the substrates with a brazing temperature equal to or lower than 1150° C., preferably in the range between 1040° C. and 1150° C., more preferably lower than 1100° C., further preferably in the range between 1080° C. and 1100° C.
There is also a need for a method allowing brazing to be conducted at a temperature equal to or lower than 1150° C., preferably between 1040° C. and 1150° C., of a moderately refractory assembly (temperature of use generally between 950° C. and 980° C.), of parts in silicon carbide-based materials irrespective of their shape and/or their size.
In particular, there is a need for a brazing method and for the associated brazing composition, allowing the brazing to be conducted at a temperature lower than 1150° C., preferably between 1040° C. and 1150° C., of silicon carbide-based parts of large size and/or of complex geometry notably having large surface areas to be brazed.
In addition, none of the methods and compositions in the prior art simultaneously meets the following criteria evidenced by the inventors which are fundamental for preparing structural components in SiC entailing moderately refractory joints:
1) the brazing composition must allow a strong bond to be obtained between the two parts in silicon carbide-based material, which necessitates a non-reactive brazing composition i.e. chemically compatible with silicon carbide, and which does not form fragile compounds therewith. However, the non-reactivity does not guarantee the forming of a strong bond since this remains unpredictable. Non-reactivity is a condition for obtaining a strong bond but it is not sufficient. For example, the Fe—Si system cited in the literature [3] is non-reactive but its mechanical strength is very weak;
2) the brazing composition must obtain good wetting of the silicon carbide and good adhesion thereto;
3) the brazing composition must be compatible with all heating devices in particular rapid and/or localised heating devices;
4) the brazing composition must allow the formation of joints having good mechanical strength;
5) the brazing composition must be formed of a limited number of elements to facilitate the preparation and implementation thereof;
6) the brazing composition must not contain costly elements such as precious metals.
Finally, the method and associated brazing composition must allow the brazing, the joining of any type of silicon carbide-based material, and must be easily adaptable to any specific silicon carbon-based ceramic.
The objective of the invention is therefore to provide a method for the joining by brazing of parts or components in silicon carbide-based materials which inter alia meets the above-cited needs, which inter alia fulfils all the requirements and criteria set forth above, which eliminates the disadvantages, defects, limitations encountered with prior art methods and which solves the problems of the prior art methods.
The objective of the invention is notably to provide a method for the joining by brazing of parts or components in silicon carbide-based materials which allows satisfactory mechanical strength of the assembly to be obtained above 500° C. and up to 950° C., even 980° C., which uses brazing temperatures equal to or lower than 1150° C., preferably in the range between 1040° C. and 1150° C., and more preferably equal to or lower than 1100° C., for example from 1080° C. to 1100° C., and which optionally allows the obtaining of joints having an excellent seal.